POROUS CARBON ELECTRODE TO IMPROVE POWER OF
PHOTOSYNTHETIC BACTERIA FUEL CELL
1
Takeyuki Moriuchi1, Keisuke Morishima1 and Yuji Furukawa2
Department of Mechanical Systems Engineering, Tokyo University of Agriculture & Technology, Japan 2
Department of Technology Management, Tokyo University of Agriculture & Technology, Japan
Abstract: This paper aims to develop a new fabrication method of porous carbon electrode in order to
improve power output of photosynthetic bacteria fuel cell. Porous carbon electrode is fabricated using
carbon micro electromechanical systems technology (C-MEMS) which is obtained from a pyrolysis of
patterned photoresist. KMPR photoresist is stirred until enough number of air micro bubbles are
generated. Then, by this bubbled photoresist, a porous photoresist electrode pattern is fabricated and after
carbonization process, a porous carbon electrode is obtained that has many micro pores of several micro
meters in diameter. Finally, micro bio-fuel cell with this porous carbon electrode is assembled and
1.4ȝW/cm2 electric output was generated which is 5 times higher than our previous work.
Key Words: micro bio-fuel cell, photosynthesis, C-MEMS, polyaniline, porous carbon
1. INTRODUCTION
In our previous work, a working principle of
micro bio-fuel cell based on photosynthesis of
microorganism called Direct Photosynthetic BioFuel Cell (DPBFC) was made clear and the
prototype has been developed[1]. This micro
DPBFC was fabricated using MEMS technology
in which the electrode was made of SU-8
photoresist and could generate a peak current
density over 36µA/cm2 and max power density of
270nW/cm2. However, these are far lower for
commercially expected use and one of the reasons
seems be caused by its high inner electrical
resistance and small reaction area of electrode.
To improve these defects, it is necessary to use
a low electrical resistance material as well as a
high-specific surface area. Therefore, carbonization of polymers to create carbon structure
called carbon-micro electromechanical systems
(C-MEMS) is highlighted which are obtained
from the pyrolysis of patterned photoresist[2]. In
addition the carbonization of photoresist easily
enables to fabricate high precise and arbitrary
shape.
In the present paper, we applied C-MEMS
technology to fabricate porous carbon electrode
with a high-specific surface area and lower
electrical resistance in order to improve the
electrical performance of micro DPBFC, and a
simple liftoff process of photoresist using gelatin
is also shown.
2. PRINCIPLE AND FABRICATION OF
POROUS CARBON ELECTRODE
2.1 Fabrication Principle of Porous Carbon
Electrode Pattern
To enhance the performance of the micro
DPBFC, improvement of the specific surface area
of an electrode is efficient, because of the reaction
area between bacteria and electrode increases and,
as a result, more current will be obtained. In the
previous work[1], micro patterned photoresist
electrode was fabricated using photolithography
process to improve the specific surface area.
Using photolithography process, it is able to
fabricate the electrode easily in arbitrary shape
with photoresist, but the surface area will be
determined and limited by an exposed mask
pattern. Therefore, in this paper, simple method to
fabricate porous carbon electrode to increase the
specific surface area more is developed using an
air bubbled photoresist.
Taking the size of bacterial cell be about 1 to 5
micro meters into account, the mean diameter of
porous shape should be about 5 micro meters.
Fabrication method of porous carbon electrode is
as follows. First, photoresist is stirred simply in
air for a certain time until enough micro bubbles
are generated inside. Porous photoresist electrode
pattern is fabricated using this bubbled photoresist.
Then, carbonization is done by pyrolysis in
vacuum or in inert gas atmosphere using furnace
and porous carbon electrode is obtained.
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2.2 Simple Liftoff Process by Gelatin Layer
To fabricate the components of micro DPBFC,
simple liftoff technique using gelatin sacrificial
layer was developed. Figure 1 shows the detail of
the liftoff process. Gelatin solution was prepared
by mixing gelatin and water at 1:7 in mass ratio
and before spin coating to wafer, gelatin solution
was heated on hot plate at 120 °C to dissolve it.
As shown in Fig.1 (1), gelatin solution is spin
coated to wafer at 500rpm for 15s and 1000rpm
for 30s and baked on a hotplate at 120 °C for 10
minutes to evaporate the water inside the solution.
Then, photoresist was spin coated onto gelatin
layer. The next step is exposing the pattern and
development. Finally, boiling this wafer in the
water, gelatin layer will dissolve easily and
removable patterned photoresist structure is
briefly obtained.
viscosity of photoresist decreases much by heat
and, from this, the length of baking time
influences the state of bubbles inside the
photoresist. Therefore, two soft bake conditions
were tried. Condition No.1 is baking on hot plate
for 5 minutes at 65 °C. Condition No.2 is baking
on hot plate for 10 minutes at 65 °C and 5 minutes
at 95 °C. After the soft bake process, expose was
done. Figure 2 shows the design of electrode
before heat treatment whose size is 24mm×18mm
with the hole whose diameter is 240ȝm. After
exposing the mask pattern for 170s, post exposure
bake was done by 65 °C for two minutes and
95 °C for 10 minutes. And development was done
by soaking into the SU-8 developer for 18 minutes
followed by developing in ultrasound bath for 5
minutes. Finally, wafer was put into boiling water
and with in about 4 minutes, gelatin layer will
dissolve and electrode patterned porous
photoresist will be obtained. Figure 3 shows the
photograph of fabricated porous photoresist
electrode pattern whose thickness was about
250ȝm. Comparing the two, condition No.1’s
surface structure is flat and also electrode pattern
is clear. At the condition No.2, the bubbles, grew
larger and got together, interfered the exposure
and some of the exposed pattern got unclear.
However, surface shows more complicated
structure than condition No.1 and it became more
three-dimensional structured.
Fig. 1: Simple liftoff process by gelatin sacrificial
layer
2.3 Fabrication of Porous Photoresist Electrode
Pattern
KMPR 1050 photoresist (KAYAKU MICROCHEM Co. Ltd) was used to fabricate bubbled
photoresist. The detail of fabrication process is as
follows. First, KMPR 1050 was heated to 95 °C
on hot plate. After that, KMPR 1050 was stirred
with plastic spoon for 10 minutes on hot plate at
95 °C until photoresist becomes creamed with the
bubbles foamed inside. Then, bubbled photoresist
was kept on a hot plate at 95 °C for 5 minutes to
defoam the large bubbles. Then, bubbled
photoresist was spin coated onto gelatin coated Si
wafer at 500 rpm for 15s and 1200 rpm for 30s.
After spin coating the bubbled photoresist on
wafer, soft bake process was done. At this process,
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Fig. 2: Design of photoresist electrode pattern
Fig. 3: Fabricated porous photoresist electrode
pattern
2.4 Fabrication of Porous Carbon Electrode
Carbonization was done in next procedures.
First, to fabricate plane electrode, photoresist
pattern was sandwiched by two carbon papers
with weights (100mg). Then, post-baked in an Ar
atmosphere (500 sccm) at 300 °C for 30 minutes,
then, heated up to 900 °C and kept for 1hour and
after that, heater was turned off to cool down the
sample to room temperature. The heating rate was
5 °C/min and the electrical furnace used in this
experiment was Advantec, FUA112DB. Figure 4
shows the photograph of fabricated porous carbon
electrode whose size was 12mm×9mm and Figure
5 shows the SEM image of electrode fabricated
with condition No.1 and condition No.2. Figure 6
shows the closeup SEM image of the single hole
of an electrode fabricated with condition No.1.
(a) Condition No.1
(b) Condition No.2
Fig. 4: Fabricated porous carbon electrode
(a) Condition No.1
(b) Condition No.2
Fig. 5: SEM images of porous carbon electrode
Fig. 6: SEM image of the single hole of an
electrode fabricated with condition No.1
From these figures, in condition No.1, large
pores (20~30ȝm) are observed over surface and
micro pores(less than 10ȝm) are observed at wall
inside the patterned hole. In condition No.2, much
larger pores are observed over the surface and
from these pores, electrode pattern became
nonuniformly.
3. FABRICATION & STRUCTURE OF
MICRO DPBFC
3.1 Working Principle of DPBFC
Cyano-bacteria is used for DPBFC. Cyanobacteria can convert H2O and CO2 to glucose
under light by its photosynthetic function and eat
this glucose for its live energy under dark by its
metabolic function. During both processes, it
generates electrons, so, a fuel cell can be realized
by gathering these electrons where both of anode
and cathode are made of polyaniline particles with
size of around 30nm or 50nm respectively, which
are known as electro-conductive material. The
generated electrons during photosynthesis and
metabolism are caught and transferred by this
polyaniline made anode[1].
3.2 Structure and Fabrication of Micro DPBFC
Figure 7 shows the structure of micro DPBFC
with pyrolyzed porous carbon electrode. The
whole size of fuel cell is 12mm×9mm×2mm
(width×depth×thickness). The chamber was
fabricated of KMPR 1050 photoresist with simple
liftoff technique using gelatin. The volume of the
chamber was about 20 micro litters.
For polymer electrolyte membrane(PEM),
Nafion®117(Dupont) was used and pretreated [1].
Next, porous carbon electrode was immersed into
the polyaniline solution (Ormecon D1024) and the
catalyst layer of polyaniline was accumulated.
The mass of the polyaniline spread on the carbon
electrode was about 1mg. Then, two electrodes
were hot pressed to one piece with 0.01 ml 10%
Nafion® dispersion solution(DE1021) and a
pretreated Nafion®117 membrane to make the
MEA at 140 °C for 3 minutes.
The assembly of micro DPBFC was done by
hot pressing at 140 °C for two minutes. Figure 8
shows a photograph of the assembled micro
DPBFC. The size of fabricated micro DPBFC was
12mm×9mm×2mm (width×depth×thickness).
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Chamber fabricated of photoresist
Porous carbon electrode
Polymer Electrolyte Membrane(PEM)
Inlet
m
Outlet
Electric pole
2mm
9m
higher electric output from the developed micro
DPBFC. The developed porous carbon electrode
is adaptable not only to the bio-fuel cell but also
to other common fuel cells.
12m
m
Fig. 7: Structure of micro DPBFC
Fig. 9: Experimental result of micro DPBFC
5. CONCLUSION
Fig. 8: Photograph of fabricated micro DPBFC
4. EXPERIMENTAL RESULTS AND
DISCUSSIONS OF MICRO DPBFC
The electrical power performance of developed
micro DPBFC was evaluated under normal room
light. In this experiment, cyanobacteria called
Synechococcus sp. was used for bio-fuel source
which was cultured for a weeks. Phosphate buffer
(pH 7.0) and BG-11 medium was added to the
anolyte. Figure 9 shows the experimental result of
micro DPBFC. The experimental results of No.1
and No.2 correspond to the electrodes fabricated
with the condition No.1 and No.2 respectively,
and the No.3 shows the experimental result for
the carbon electrode fabricated without bubble
treatment in the previous research[3]. It is seen
that the porous carbon electrode showed 5 times
higher performance than that of normal carbon
electrode. It is considered that the more the
surface area of an electrode increased, the higher
the contact area between bacteria and electrode
took place, hence the more current was extracted.
Actually, the specific surface area of porous
carbon electrode fabricated in the condition No.1
and No.2 should be at least three and five times
higher than that of the normal carbon electrode
according to the approximated geometrical
calculation. In this way, it is verified to apply the
porous carbon electrode be advantageous to obtain
18
A simple fabrication method using bubbled
photoresist to fabricate porous photoresist
structure was developed. Applying C-MEMS
technology, a porous carbon electrode was
fabricated and the effectiveness of applying this
porous carbon electrode to micro DPBFC was
verified from the performance evaluation of
electrical power. Additionally, a novel and simple
lift off process using gelatin layer as a sacrificial
layer was developed to fabricate a stand alone
micro structure and was applied to fabricate
porous carbon electrode, and of course, the
process is conceivably applied to many MEMS
components manufacturing too.
REFERENCES
[1] Y. Furukawa et al., Design principle and
prototyping of a direct photosynthetic/
metabolic biofuel cell (DPMFC), J. Microelectromech. Syst., vol. 16, pp.220-225, 2006.
[2] B. Y. Park and M. J. Madou, Miniature PEM
Fuel Cell with Pyrolyzed Carbon
Microfluidic Plates, in Digest Tech. Papers
PowerMEMS 2006 Conference, Berkeley,
Nov. 29-Dec. 1, 2006, pp.105-108.
[3] T. Moriuchi et al., Improvement of power
capability applying pyrolyzed carbon photoresist electrode for Micro Direct Photosynthetic/metabolic Bio-Fuel Cell (DPBFC),
in Digest Tech. Papers ASPEN, Gwangju,
Nov. 6-9, 2007.